1. Field of the Invention
The present invention relates to a method of fabricating an optical waveguide structure by use of light and a photo-curable liquid resin. More particularly, the present invention relates to a method of fabricating an optical waveguide structure by use of light and a mixture of two types of photo-curable liquid resins having different curing initiation wavelengths and different refractive indexes, in which the core portion of the optical waveguide structure is formed through curing of one photo-curable liquid resin, and the cladding portion of the optical waveguide structure is formed through curing of both the photo-curable liquid resins.
The present invention further relates to a method of fabricating an optical waveguide structure in which an optical fiber is dipped into the above-described liquid mixture in order to form an optical waveguide structure which continues from the optical fiber and is excellent in terms of straightness and parallelism.
The present invention is applicable to optical connectors for interconnection, optical splitters, and optical mixers which are used in optical communications and which are inexpensive and involve lowered loss.
2. Description of the Related Art
Recently, there has been widely noticed a technique for forming an optical waveguide at a tip end of an optical fiber by use of a photo-curable liquid resin. Japanese Patent Application Laid-Open No. 4-165311 discloses an exemplary method for fabricating an optical waveguide structure. Briefly, in a first step, one end of an optical fiber is dipped into a photo-curable liquid resin containing fluorine-based monomers. In a subsequent step (second step), light having a wavelength suitable for curing the liquid resin is radiated from the tip end of the fiber.
When a laser beam having, for example, a wavelength close to a UV range or a shorter wavelength is caused to radiate from the tip end of the optical fiber, a portion of the photo-curable liquid resin adjacent to the tip end cures through photopolymerization. Thus, a so-called core portion is formed at the tip end of the fiber in accordance with the power distribution of the laser beam. The formation of the core portion enables the laser beam to propagate farther, so that the core portion is extended. As a result, an optical waveguide is formed.
In a third step, the optical fiber is removed from the photo-curable liquid resin and is subjected to cleaning or a like process in order to remove a portion of the photo-curable liquid resin which remains in an uncured state. Subsequently, in a fourth step, the core portion is coated with light-transmissive resin in order to protect the core portion from dust and prevent damage to the core portion. In a final, fifth step, the tip end surface of the thus-formed core portion is ground in order to form a light output surface of the waveguide.
As described above, an optical waveguide continuous from an optical fiber is formed in five steps.
However, when such an optical waveguide is formed by the conventional method, a resultant optical waveguide meanders, while the cross-sectional area of the path increases gradually. The term xe2x80x9cmeanderxe2x80x9d means that the radius periodically changes along a Z-axis or optical axis. This phemomenon occurs due to mismatch in refractive index between the core portion of the optical fiber and a photo-curable liquid resin. As a result, there is formed a graded-index-type optical waveguide having an widened beam aperture.
In such a graded-index-type optical waveguide, light meanders in accordance with refractive index. That is, the focal distance changes depending on the length of the waveguide. Therefore, in the final step for grinding the end surface, the grinding amount must be determined while the focal distance is measured. This results in an extremely high fabrication cost.
In addition, the waveguide length of the core portion formed by the conventional method is limited to 8.5 mm. When end surface treatment is performed, the waveguide length decreases further. Therefore, such a conventional method is difficult to apply to cases in which a split mirror is inserted in a waveguide in order to fabricate an optical splitter or combiner, although it can be applied to connectors for connecting optical fibers.
There has also been reported that a tapered optical waveguide is formed at the tip end of an optical fiber. Formation of such a tapered optical waveguide also stems from mismatch in refractive index between the core portion of the optical fiber and a photo-curable liquid resin. When such a tapered optical waveguide is used in an optical combiner or splitter, its loss increases due to an increase in cross-sectional area.
Further, the above-described method has a drawback in that if the cladding layer is cured as is, the refractive index of the cladding layer becomes the same as that of the core portion. Accordingly, in order to obtain a step-index-type optical waveguide, an additional step for replacing the material for fabricating the cladding portion with any other material is required, resulting in deterioration in productivity.
The present invention was accomplished in order to solve the above-described problems, and an object of the present invention is to provide a method of fabricating an optical waveguide structure which facilitates formation of core and cladding portions by use of a liquid mixture of two types of photo-curable liquid resins and which enables the core portion to extend straight. The term of xe2x80x9ca liquid mixture of two types of photo-curable liquid resinsxe2x80x9d herein means that the liquid mixture should contain a plurality of photo-curable liquid resins which are not identical in curing initiation wavelength and refractive index, as hereinafter described. Hence, the photo-curable liquid resins to be used are not necessarily of two kinds.
Another object of the present invention is to provide a method of fabricating an optical waveguide structure in which the refractive index of the liquid mixture is adjusted in accordance with an optical fiber to be used to thereby enable formation of an optical waveguide which extends straight from a light output end of an optical fiber, regardless of the type of the optical fiber.
Still another object of the present invention is to provide a method of fabricating an optical waveguide structure which can greatly reduce assembly cost and parts cost in order to render the optical waveguide structure inexpensive.
In order to achieve the above objects, according to a first aspect of the present invention, there is provided a method of fabricating an optical waveguide structure in which light of a predetermined wavelength is introduced into a photo-curable liquid resin in order to cure the photo-curable liquid resin along an opticalaxis to thereby form an optical waveguide structure continuing from an area through which the light is introduced. The photo-curable liquid resin is a liquid mixture of a first photo-curable liquid resin and a second photo-curable liquid resin having a curing initiation wavelength shorter than that of the first photo-curable liquid resin. A light beam of a wavelength capable of curing only the first photo-curable liquid resin is radiated into the liquid mixture so as to form a core portion having a rod-like shape. Subsequently, light of a wavelength capable of curing both the first and second photo-curable liquid resins is radiated into the liquid mixture from an area surrounding the liquid mixture so as to form a hardened cladding portion surrounding the core portion. The core portion has a refractive index greater than that of the cladding portion.
In the fabrication method according to a second aspect of the present invention, the refractive index of the first photo-curable liquid resin after curing is greater than the refractive index of the liquid mixture.
In the fabrication method according to a third aspect of the present invention, the beam of light having the predetermined wavelength is radiated from a tip end of an optical fiber dipped into the liquid mixture, and the optical waveguide structure is formed so as to be continuous from the tip end of the optical fiber.
In the fabrication method according to a fourth aspect of the present invention, the optical fiber is a step-index-type optical fiber whose refractive index changes stepwise at the boundary between the core portion and the cladding portion. In this case, the refractive index nC1 of the liquid mixture is adjusted so as to satisfy the following conditional equation (5):
nf12xe2x88x92nf22xe2x89xa6nA22xe2x88x92nC12xe2x80x83xe2x80x83(5)
where nf1 is the refractive index of the core portion of the optical fiber, nf2 is the refractive index of the cladding portion of the optical fiber, and nA2 is the refractive index of the core portion of the optical waveguide structure.
In the fabrication method according to a fifth aspect of the present invention, the optical fiber is a graded-index-type optical fiber whose refractive index is graded in the radial direction in accordance with a predetermined function. In this case, the refractive index nC1 of the liquid mixture is adjusted such that the diameter 2aW of the core portion of the formed optical waveguide structure satisfies the following conditional equation (6):
2aW=2af[1/(2xcex94)xc2x7(nA22xe2x88x92nc12)/nA22]1/pxe2x80x83xe2x80x83(6)
where xcex94=(nf12xe2x88x92nf22)/(2nf12), nf1 is the maximum refractive index of the core portion of the optical fiber, i.e., refractive index of the center of the core, 2a f is the diameter of the core portion, nf2 is the refractive index of the cladding portion of the optical fiber, nA2 is the refractive index of the core portion of the optical waveguide structure, and p is an integer.
According to sixth and seventh aspects of the present invention, there is provided a method of fabricating an optical waveguide structure in which a tip end of an optical fiber is dipped into a photo-curable liquid resin, and light of a predetermined wavelength is radiated from the tip end in order to cure the photo-curable liquid resin along an optical axis to thereby form an optical waveguide continuing from the tip end of the optical fiber. In the fabrication method according to the sixth aspect, the optical fiber is a step-index-type optical fiber whose refractive index changes stepwise at the boundary between the core portion and the cladding portion; and the refractive index nC1 of the photo-curable liquid resin is adjusted so as to satisfy the following conditional equation (7):
nf12xe2x88x92nf22xe2x89xa6nA22xe2x88x92nC12xe2x80x83xe2x80x83(7)
where nf1 is the refractive index of the core portion of the optical fiber, nf2 is the refractive index of the cladding portion of the optical fiber, and nA2 is the refractive index of the formed optical wave guide structure.
In the fabrication method according to the seventh aspect, the optical fiber is a graded-index-type optical fiber whose refractive index is graded in the radial direction in accordance with a predetermined function; and the refractive index nC1 of the photo-curable liquid resin is adjusted such that the diameter 2aW of the formed optical waveguide structure satisfies the following conditional equation (8):
2aw=2af [1/(2xcex94)xc2x7(nA22xe2x88x92nc12)/nA22]1/pxe2x80x83xe2x80x83(8)
where xcex94=(nf12xe2x88x92nf22)/(2nf12), nf1 is the maximum refractive index of the core portion of the optical fiber, i.e., refractive index of the center of the core, 2af is the diameter of the core portion, nf2 is the refractive index of the cladding portion of the optical fiber, nA2 is the refractive index of the formed optical waveguide structure, and p is an integer.
The method of fabricating an optical waveguide structure according to the first aspect of the present invention utilizes a liquid mixture of two types of photo-curable liquid resins; i.e., a liquid mixture of a first photo-curable liquid resin and a second photo-curable liquid resin having a curing initiation wavelength shorter than that of the first photo-curable liquid resin. The refractive index of the first photo-curable liquid resin after curing is set greater than the refractive index of the liquid mixture after curing. The liquid mixture is placed in, for example, a transparent container having the shape of a rectangular parallelepiped.
A beam of light having a wavelength xcexW (xcex2 less than xcexW less than xcex1) capable of curing only the first photo-curable liquid resin is radiated into the liquid mixture, where xcex1 is the curing initiation wavelength of the first photo-curable liquid resin, and xcex2 is the curing initiation wavelength of the second photo-curable liquid resin. The wavelength at which each photo-curable liquid resins starts curing is smaller than the curing initiation wavelength of each resins. The beam of light having the wavelength xcexW may be generated by use of, for example, a short-wavelength laser such as a He-Cd laser.
As result, only the first photo-curable liquid resin of the liquid mixture cures through photopolymerization, thereby forming a straight core portion. At this time, the liquid mixture of two types of photo-curable liquid resins remains around the circumferential surface of the core portion.
Subsequently, light having a wavelength xcexC (xcexC less than xcex2) capable of curing both the first and second photo-curable liquid resins is radiated into the liquid mixture from an area surrounding the liquid mixture, by use of, for example, a UV lamp. As a result, the remaining liquid solidifies through photopolymerization, so that a cladding portion is formed around the core portion.
Through the above-described setting of the refractive index of the liquid mixture, the refractive index of the core portion becomes greater than that of the cladding portion. That is, a step-index-type optical waveguide structure is formed.
As described above, a step-index-type optical waveguide structure having core and cladding portions is formed through irradiation of light in two steps. Accordingly, the fabrication method is considerably efficient.
The transparent container for accommodating the liquid mixture may have an arbitrary shape. This enables the cladding portion to be formed into an arbitrary shape in accordance with, for example, the shape of a target product or component. That is, the cladding portion can be fixed directly to a product or component. Accordingly, the optical waveguide structure fabricated by the present method is very convenient.
Since the optical waveguide structure is integrally fabricated through irradiation of a light beam having a wavelength xcexW and light having a wavelength xcexC, the optical waveguide structure can be fabricated at low cost.
As described in Japanese Patent Application No. 10-152157, the above-described steps may be performed after an optical element such as a half mirror is placed in the above-described container in order to fabricate an optical splitter in which an optical waveguide is integrated with the half mirror.
It has been known that when the optical waveguide structure is deformed, the phase of a light wave changes. Since the above-described fabrication method enables formation of the cladding portion into an arbitrary shape, a physical phenomenon such as stress, an electric field, a magnetic field, or an ultrasonic wave can be applied to the optical waveguide structure with ease.
Therefore, stress can be easily applied to the optical waveguide structure in various manners; i.e., the phase of a light wave can be easily changed in various manners. Accordingly, an optical element such as a phase-modulation element can be formed.
That is, the above-described fabrication method serves as a fundamental technique for forming fundamental structures of a various types of optical elements having optical waveguides.
In the fabrication method according to the second aspect of the present invention, the refractive index of the first photo-curable liquid resin after curing is rendered greater than the refractive index of the liquid mixture. That is, upon irradiation of the light of the wavelength xcex1, the first photo-curable liquid resin hardens, so that the refractive index of the first photo-curable liquid resin becomes higher than the refractive index of the liquid mixture. Thus, a step-index-type optical waveguide is formed in the liquid mixture.
Since the formed waveguide is of the step-index-type, the incident light causes total reflection, so that the waveguide is extended efficiently. Therefore, the light radiated into the liquid mixture is not limited to a laser beam which propagates straight. For example, a UV beam may be used. In this case, the UV beam is radiated into the liquid mixture at an angle which causes total reflection. Therefore, in the fabrication method of the second aspect, any of various types of light sources may be used.
In the fabrication method according to the third aspect of the present invention, a tip end of an optical fiber is dipped into the liquid mixture, and a beam of light having the predetermined wavelength is radiated from the tip end of the optical fiber. The beam of light having the predetermined wavelength may be a short-wavelength laser beam.
The short-wavelength laser beam causes the first photo-curable liquid resin to cure along the optical axis through photopolymerization. Thus, the core portion of the optical waveguide structure is formed straight such that the core portion maintains close contact with the core portion of the optical fiber and is continuous therefrom. Therefore, it becomes unnecessary to align the optical axis of the optical waveguide structure and that of the optical fiber.
Further, the tip end of the optical fiber is firmly fixed with the cladding portion of the optical waveguide structure through irradiation of light having the above described wavelength xcexC. Accordingly, the optical waveguide structure can be disposed with a high degree of freedom and can be handled with ease. Accordingly, the optical waveguide structure is highly convenient.
In the fabrication method according to the fourth aspect of the present invention, the optical fiber is a step-index-type optical fiber whose refractive index changes stepwise at the boundary between the core portion and the cladding portion; and the refractive index nf1 of the center of the core portion of the optical fiber, the refractive index nf2 of the cladding portion of the optical fiber, the refractive index nA2 of the core portion of the optical waveguide structure, and the refractive index nC1 of the liquid mixture satisfy conditional equation (5).
This condition equation represents conditions under which all light which has propagated within the step-index-type optical fiber while causing total reflection causes refraction at the boundary between the core portion of the optical fiber and the core portion of the optical waveguide structure, and the refracted light propagates within the core portion of the optical waveguide structure while causing total reflection.
The refractive index of the liquid mixture is adjusted so as to satisfy condition equation (5). The optical waveguide structure may be formed even when condition equation (5) is not satisfied. In this case, however, the shape of the optical waveguide structure becomes nonuniform, and transmission loss due to leakage of light increases. When condition equation (5) is satisfied, all light having propagated through the optical fiber is refracted at the above-described boundary and propagates within the core portion of the optical waveguide structure while causing total reflection.
The total reflection of light within the core portion of the optical waveguide structure enables continuous formation of the core portion. That is, the core portion of the step-index-type optical fiber is extended straight in order to form the optical waveguide structure. Thus, there is fabricated a straight optical waveguide structure connected directly to the tip end of the step-index-type optical fiber.
In the fabrication method according to the fifth aspect of the present invention, the optical fiber is a graded-index-type optical fiber whose refractive index is graded in the radial direction in accordance with a predetermined function; and the refractive index nf1 of the core portion of the optical fiber, the diameter 2af of the core portion, the refractive index nf2 of the cladding portion of the optical fiber, the refractive index nA2 of the core portion of the optical waveguide structure, the refractive index nC1 of the liquid mixture, and the diameter 2aW of the core portion of the formed optical waveguide structure satisfy the above-described conditional equation (6). In equation (6), p is an integer.
Equation (6) indicates that the diameter 2af of the core portion of the optical waveguide structure can be controlled by means of proper selection of the refractive index nC1 of the liquid mixture. The refractive index nC1 of the liquid mixture can be adjusted through adjustment of the mixing ratio between the two types of photo-curable liquid resins.
The refractive index nC1 of the liquid mixture is selected to satisfy equation (6). Therefore, light which has propagated straight in the vicinity of the optical axis of the optical fiber while being subjected to refraction is extracted though a reduced light output surface. The thus-extracted light propagates straight, so that a step-index-type optical waveguide structure is formed in the liquid mixture. Accordingly, the fabrication method of the present aspect can be applied to a graded-index-type optical fiber used for high-speed communications.
In the fabrication method according to the sixth aspect, a tip end of a step-index-type optical fiber is dipped into a photo-curable liquid resin, and light of a predetermined wavelength is radiated from the tip end of the optical fiber. The light of a predetermined wavelength may be a short-wavelength laser beam. The short-wavelength light continuously causes photopolymerization of the photo-curable liquid resin along the optical axis.
Thus, a rod-shaped optical waveguide (core portion) is formed such that the optical waveguide maintains close contact with the core portion of the optical fiber and is continuous therefrom. There, in this case as well, alignment between the optical axis of the optical fiber and the optical axis of the optical waveguide is not required.
The first to fifth aspects of the present invention are characterized in that core and cladding portions of an optical waveguide structure are formed through irradiation of light in two steps, and the core and cladding portions are generally referred to as an xe2x80x9coptical waveguide structure.xe2x80x9d In contrast, the sixth and seventh aspects of the present invention are characterized in that a straight optical waveguide structure (including a core portion only) is formed. by use of a single type of a photo-curable liquid resin. Therefore, in the sixth and seventh aspects, the term xe2x80x9coptical waveguide structurexe2x80x9d has the same meaning as that of the term xe2x80x9ccore portion.xe2x80x9d
In the above-described fabrication method, the refractive index nf1 of the core portion of a step-index-type optical fiber, the refractive index nf2 of the cladding portion of the optical fiber, the refractive index nA2 of the optical waveguide structure formed in the photo-curable liquid resin, and the refractive index nC1 of the photo-curable liquid resin satisfy the above-described conditional equation (7).
This condition equation represents conditions under which all light which has propagated within the step-index-type optical fiber while causing total reflection causes refraction at the boundary between the core portion of the optical fiber and the optical waveguide structure, and the refracted light propagates within the optical waveguide structure while causing total reflection.
The refractive index of the photo-curable liquid resin is adjusted so as to satisfy condition equation (7). Therefore, light having propagated through the optical fiber is transmitted to the optical waveguide structure, so that the light extends the optical waveguide structure while causing total reflection.
That is, according to the fabrication method of the present invention, the core portion of the step-index-type optical fiber is extended straight in order to form the optical waveguide structure, which is excellent in terms of straightness and parallelism.
When a liquid whose refractive index increases after cure and satisfies equation (7) is employed, a single type of photo-curable liquid resin may be used as described above. Further, when, as in the third aspect, the refractive index of one photo-curable liquid resin which is selectively cured in order to form a core portion of the optical waveguide structure is made higher than that of the other photo-curable liquid resin which does not cure during the formation of the core portion, the difference between the refractive index of the core portion after curing and that of the liquid mixture can be made greater, so that the liquid that satisfies equation (7) can be prepared easily. The liquid may be formed of a single type of a photo-curable liquid resin or a plurality of types of photo-curable liquid resins. In this case, the optical waveguide structure is surrounded by an uncured photo-curable liquid resin (liquid). In actual practice, any type of medium may exist around the optical waveguide structure. For example, gas, another liquid, or a solid material may exist around the optical waveguide structure, insofar as the selected medium has a refractive index lower than that of the optical waveguide structure.
For example, before use, the optical waveguide structure is removed from the photo-curable liquid resin and is cleaned. As a result, the circumference of the optical waveguide structure is surrounded by air serving as a cladding portion, so that the refractive index of the core portion becomes greater than that of the cladding portion. Therefore, total reflection conditions are satisfied, and the optical waveguide structure serves as a step-index-type optical waveguide structure producing a reduced transmission loss. When the core potion is surrounded by liquid, an optical waveguide structure having a liquid cladding portion is obtained. When the core potion is surrounded by a solid material, an optical waveguide structure having a solid cladding portion is obtained.
Further, the above-described optical waveguide structure is flexible and has a rod-like shape. Therefore, the optical waveguide structure can be disposed in direct contact with a light-emitting window of a semiconductor laser element or a light-receiving element having a small light receiving area, wherein the semiconductor laser element and the light-receiving element are formed on a semiconductor substrate. Accordingly, the fabrication method of the present embodiment enables fabrication of an optical waveguide structure convenient for light input and light output.
In the fabrication method according to the seventh aspect, the optical fiber dipped into the photo-curable liquid resin is a graded-index-type optical fiber whose refractive index is graded in the radial direction in accordance with a predetermined function; and the refractive index nf1 of the core portion of the optical fiber, the diameter 2af of the core portion, the refractive index nf2 of the cladding portion of the optical fiber, the refractive index nA2 of the optical waveguide structure, the refractive index nC1 of the photo-curable liquid resin, and the diameter 2aW of the formed optical waveguide structure satisfy the above-described conditional equation (8). In equation (8), p is an integer.
Equation (8) indicates that the diameter 2aW of the optical waveguide structure can be controlled by means of proper selection of the refractive index nC1 of the liquid mixture. The refractive index nC1 of the liquid mixture can be adjusted through adjustment of the mixing ratio between the two types of photo-curable liquid resins.
The refractive index nC1 of the photo-curable liquid resin is selected so as to satisfy equation (8). Therefore, light which has propagated straight in the vicinity of the optical fiber while being subjected to refraction is extracted though a reduced light output area. The thus-extracted light propagates straight, so that a step-index-type optical waveguide structure is formed in the photo-curable liquid resin. Accordingly, the fabrication method of the present aspect can be applied to a graded-index-type optical fiber used for high-speed communications.
In the fabrication method of the seventh aspect as well, the liquid may be formed of a single type of a photo-curable liquid resin or a plurality of types of photo-curable liquid resins.